This invention relates to a combustion system and method for a coal-fired furnace and, more particularly, to such a system and method which utilizes coal as the primary fuel and combusts a coal-air mixture.
In a typical coal-fired furnace, particulate coal is delivered in suspension with primary air from a pulverizer, or mill, to the coal burners, or nozzles, and a secondary air is provided to supply a sufficient amount of air to support combustion. After initial ignition, the coal continues to burn due to local recirculation of the gases and flame from the combustion process.
In these types of arrangements, the coal readily burns after the furnace has been operating over a fairly long period of time. However, for providing ignition flame during startup and for warming up the furnace walls, the convection surfaces and the air preheater; the mixture of primary air and coal from conventional main nozzles is usually too lean and is not conducive to burning under these relatively cold circumstances. Therefore, it has been the common practice to provide oil or gas fired ignitors and/or guns for warming up the furnace walls, convection surfaces and the air preheater, since these fuels have the advantage of a greater ease of ignition and, therefore, require less heat to initiate combustion. The ignitors are usually started by an electrical sparking device or swab, and the guns are usually lit by an ignitor or by a high energy or high tension electrical device.
Another application of auxiliary fuels to a coal-fired furnace is during reduced load conditions when the coal supply, and, therefore, the stability of the coal flame, is decreased. Under these conditions, the oil or gas ignitors and/or guns are used to maintain flame stability in the furnace and thus avoid accumulation of unburned coal dust in the furnace.
However, in recent times, the foregoing advantages of oil or gas fired warmup and low load guns have been negated by the increasing costs and decreasing availability of these fuels. This situation is compounded by the ever-increasing change in operation of coal-fired nozzles from the traditional base-loaded mode to that of cycling, or shifting, modes which place even heavier demands on supplemental oil and gas systems to support these types of units.
To alleviate these problems, it has been suggested to form a dense phase particulate coal by separating air from the normal mixture of pulverized coal and air from the mill and then introducing the air into a combustion supporting relation with the resulting dense phase particulate coal as it discharges from its nozzle. However, this has required very complex and expensive equipment externally of the nozzle to separate the coal and transport it in a dense phase to the nozzle.